There is an increasing demand for the use of biomass for partly replacing petroleum resources for the synthesis of fuels. The use of bioethanol for the synthesis of base stocks for fuels therefore arouses a great interest. The production of liquid hydrocarbons on acid solids has been mentioned by some authors during ethanol conversion reactions (H. Van Bekkum et al., Applied Catalysis, 3 (1982)). However, they took no interest in the optimization of the gas oil yield.
The reaction at the root of the method of converting ethanol to a base stock for diesel fuel is dehydration-oligomerization of the ethanol in a single stage according to Equation (1) below:2C2H5OH→2CH2═CH2+2H2O→oligomerization/cyclization(aromatics, paraffins, olefins, etc.)  (1)
It is well known that dehydration of ethanol occurs quite easily on acid solids of low acidity at temperatures above 300° C. and at atmospheric pressure. The reaction products are then mainly water and ethylene, ethylene being obtained with a selectivity above 96%. The most commonly used catalysts are silica-aluminas, unprocessed zeolites (ZSM-5) or zeolites modified by steaming, or asbestos-derived zeolites. The use of a triflic acid-treated ZSM-5 zeolite (R. Le Van Mao et al., The Bioethanol-to-Ethylene (B.E.T.E.) Process, Applied Catalysis, 48 (1989)), or of a microporous niobium silicate AM-11 (P. Brandao et al., Dehydration of Alcohols by Microporous Niobium Silicate AM-11, Catalysis Letters, 80, 3-4 (2002)) have also been mentioned in the prior art. A relatively old study mentions the production of aromatics up to 50% from ethanol over ZSM-5 at temperatures above 260° C. For lower temperatures, only the formation of ethylene is mentioned. The presence of water in the ethanol feedstock seems to promote the formation of aromatics, in opposition to the conclusions of A. T. Aguayo et al. (J. Chem. Technol. And Biotechnol., 77 (2002)). The presence of water would also have the effect of limiting the deactivation of the catalyst(5,6). On the other hand, for temperatures above 450° C., there is a risk of catalyst dealumination.
Ethylene oligomerization requires high pressures, generally ranging between 2 and 4 MPa, but lower temperatures, generally between 20° C. and 200° C. The catalysts used are in most cases transition metals deposited on silica-alumina type supports, zeolites (ZSM-5) or mesoporous solids (MCM-41) as described by V. Hulea et al., J. Catal., 225 (2004).
Few authors have reported the dehydration-oligomerization of ethanol in a single stage. The few studies mentioned (S. Sivasanker et al., S. Assam Science Society, 36(3), (1994) or D. R. Whitcraft et al., Ind. Eng. Chem. Process Dev., 22, (1983)) thus show the production of gasoline cuts by reaction of ethanol at high pressure and temperature over ZSM-5. However, the yields obtained are rather low and the heavy cut (Tboiling>220° C.) represents a small percentage (<3%). The formation of aromatics is mentioned: it depends on the pressure and on the Si/Al ratio of the zeolite. Advanced kinetic studies concerning the conversion of aqueous ethanol over H-ZSM-5 zeolites to hydrocarbons were carried out by A. T. Aguayo et al. as mentioned above. However, the reactions occur at atmospheric pressure and at high temperature; the products obtained are not detailed, but their molecular mass is low (C5+).
The reaction of converting ethanol to produce hydrocarbons (dehydration-oligomerization in a single stage) has mainly been studied on ZSM-5 zeolite (M. M. Chang et al., “The Conversion of Methanol and Other O-Compounds to Hydrocarbons over Zeolite Catalysts”, J. Catal. 47, 249-259). The main goal was to produce gasoline type effluents, but no author attempted to optimize the yield in liquid hydrocarbons with a boiling point temperature above 150° C.
K. G. Bhattacharyya et al. (“Production of Hydrocarbons from Aqueous Ethanol over HZM-5 under High Pressure”, Journal of Assam Science Society 36(3), pp 177-188 (1994)) are the only ones who took an interest in the results in terms of diesel cut production. However, they have not tried to optimize the operating conditions or the catalyst. The tests were carried out at 3 MPa and 400° C. on an H-ZSM-5 zeolite of Si/Al ratio 103, i.e. of relatively low acidity. The diesel cut fraction (270-370° C.) obtained is then only 0.6%. The operating conditions vary a lot from one study to the next but the pressure generally favours the formation of liquid products (>C5+), temperatures above 350° promote the oligomerization of ethylene, the primary product from the reaction of ethanol from 300° C. Above 350° C., the formation of aromatics becomes significant, notably over H-ZSM-5. This catalyst is by far the most stable of all the zeolites studied (mordenite, Y or beta).
The addition of metals by ionic exchange has been studied by J. F. Schulz et al. (“Conversion of Ethanol over Metal-exchanged Zeolites”, Chem. Eng. Technol. 16 (1993) 332-337), who showed the influence of nickel on the formation of aromatics. According to them, addition of this metal allows to stabilize the aluminium sites of the zeolites, thus preventing crystallinity loss. A low Si/Al ratio of the catalyst favours the formation of aromatics. According to Valle B. et al. (“Effect of Nickel Incorporation on the Acidity and Stability of HZSM-5 Zeolite in the MTO Process”, Catalysis Today 106 (2005) 118-122), in the case of the “Methanol to Olefins” process, which requires a high temperature and takes place in the presence of a large amount of water, addition of nickel by impregnation allows the H-ZSM-5 zeolite to be stabilized. The presence of nickel causes the acidity of the catalyst (strength and number) to fall. However, a 1% nickel content allows the catalyst to be made regenerable without activity loss, unlike the parent solid that deactivates. Machado et al. (“Obtaining Hydrocrabons from Ethanol over Iron-modified ZSM-5 Zeolites”, Fuel 84, 2064-2070) modified a ZSM-5 of Si/Al ratio 20 (previously exchanged to obtain the protonic form) by impregnation with Fe(NO3)3, 9H2O) or by ion exchange with FeCl3, 6H2O.
The ZSM-5 zeolite is considered to be microporous since the major part of its pores is smaller than 20 Å.
On the other hand, some authors have compared the dehydration and oligomerization mechanisms of ethanol and of methanol. Derouane et al (J. Catal, 53, 40-55 (1978)) notably showed that the behaviour of these two alcohols in the conversion reaction over acid solids was different. Thus, under identical conditions, at 250° C., more than 98% of the ethanol is converted to ethylene whereas the main product detected from methanol (74%) is dimethyl ether. Espinoza et al (App. Catal, 6, 11-26 (1983)) show that 93% of the ethanol is converted to ethylene at 380° C. and 49% of the methanol is converted to C5+.
The mechanism is obviously different for the two alcohols: in fact, methanol first has to react with itself to form dimethyl ether by eliminating a first water molecule, then the elimination of a second water molecule allows to obtain the ethylene that can thereafter grow via the formation of a longer ether (by addition of a methoxy group on a C2 carbocation thus leading to the formation of propylene), or by reaction with another ethylene molecule.
Above 300° C., the conversion of ethanol predominantly goes through the formation of ethylene (directly produced by intramolecular dehydration of the alcohol or via the diethyl ether), the growth of the chains thus occurring via carbocationic intermediates (formation of even chains).
In conclusion, the state of the art as regards patent applications for a method allowing conversion of ethanol to a base stock for diesel fuel by means of dehydration-oligomerization in a single stage comprises no pertinent anteriority. The scientific literature essentially took an interest in the conversion of ethanol to an aromatic base, insofar as the diesel fraction obtained did not exceed 1% by mass.